1. Field of the Invention
The present invention relates to a light emitting diode drive circuit having a DC-DC converter circuit for driving a light emitting diode (hereinafter referred to as “LED”) at a constant current.
2. Description of the Related Art
As a common DC-DC converter circuit for driving an LED at a constant current, a booster circuit shown in FIG. 2 is widely known.
A smoothing capacity 104 is connected between a rectifying device 103 and a ground (GND). In addition, a switching element 105 is connected between a connecting point at which an inductor 102 and the rectifying device 103 are connected to each other, and the ground. An LED 108 and a current sense element 107 are connected in series to each other between a connecting point at which the rectifying device 103 and the smoothing capacity 104 are connected to each other, and the ground. Further, an output of the current sense element 107 is connected to a control circuit 106, and an output of the control circuit 106 is connected to the switching element 105.
The control circuit 106 monitors a voltage of the current sense element 107, and controls short-circuiting and open-circuiting of the switching element 105, thereby controlling a current caused to flow through the LED 108 at a proper value to cause the LED to emit light properly. In other words, in order to cause a proper current to flow through the LED 108, a voltage of the smoothing capacity 104 is controlled so that the voltage becomes a sum of a forward voltage when a proper current is caused to flow through the LED 108, and a voltage generated when a proper current is caused to flow through the current sense element 107.
However, in driving an LED in the boost DC-DC converter circuit shown in FIG. 2, there is a problem in that, when a power supply voltage is increased to be higher than a forward voltage of the LED to be driven, a current flowing through the LED cannot be controlled.
In other words, provided that a voltage generated at the time when a current is caused to flow through the rectifying device 103, is set to 0 V, when a voltage of a power supply 101 exceeds a sum of a forward voltage generated due to a proper current caused to flow through the LED 108 and a voltage generated due to a proper current caused to flow through the current sense element 107, a current caused to flow through the LED 108 and the current sense element 107 each are increased to be larger than a proper value. As a result, the LED emits light excessively, and at worst, the LED may break down.